The present invention relates to a transmission system having a main transmission and a sub transmission.
In a known transmission system which has a main transmission and a sub transmission, e.g., a four-wheel drive automatic transmission system, the sub transmission is automatically changed over to a lower speed mode only when the vehicle running speed is determined to be lower than a predetermined speed. To achieve this operation, a governor valve which produces the oil pressure in accordance with the rotational speed of the main transmission is provided either on the output shaft of the main transmission or on the input shaft of the sub transmission, and the speed change mechanism of the sub transmission is controlled by the governor pressure produced by the governor valve.
The above-mentioned conventional transmission system employs a governor valve as a main transmission output shaft speed sensing means. Since the governor valve converts the rotational speed of the output shaft of the main transmission to a hydraulic pressure, it is impossible for a known transmission system to incorporate an electric controller or a computer in which the shift change mechanism and the speed change mechanism are automatically controlled by means of the speed change signal which is determined by the computer in accordance with the state of the running of the vehicle in terms of such factors as vehicle running speed, throttle opening, the rotational speed of the output shaft of the main transmission and so forth. It has also been proposed that a speed sensor be provided which converts the speed of rotation of the output shaft of the sub transmission to an electric signal so as to detect the rotational speed corresponding to the rotational output of the sub transmission. In the case that the shift control of the main transmission is performed by means of the rotational speed of the output shaft of the sub transmission, when the speed mode of the sub transmission is changed, for example, from a high speed to a low speed, the rotational speed of the output shaft of the main transmission is raised in accordance with the gear ratio. However, the rotational speed of the output shaft of the sub transmission remains the same if the vehicle running speed is kept the same, and the current transmission gear selected by the main transmission is not shifted up even if the rotational speed of the engine is high. Similarly, when the speed mode of the sub transmission is changed from the low speed to the high speed, the rotational speed of the output shaft of the main transmission is lowered in accordance with the gear ratio. However, the rotational speed of the output shaft of the sub transmission remains the same if the vehicle running speed is kept the same, and the current transmission gear selected by the main transmission is not shifted down, even if the rotational speed of the engine is low. It has also been proposed that the shift control of the main transmission be effected by employing the rotational speed of the output shaft of the sub transmission which is obtained by multiplying the detected rotatonal speed by the gear ratio of the current speed mode of the sub transmission. In this method, however, it is impossible to accurately obtain the rotational speed of the output shaft during the time when the speed mode of the sub transmission is being changed, thereby making it difficult to control the main transmission accurately.